When Vermonter Nancy Heydinger witnessed the bomb blasts minutes after completing this year’s Boston Marathon, she didn’t react like most other runners.

Instead, she morphed into a mother.

Four hours earlier, Heydinger had stood at the start with her college-age daughter, who ran ahead of her with plans to reunite at the finish. But when the 52-year-old reached the end of the 26-mile race at 2:41 p.m., she encountered something else.

“I was really excited, smiling and happy — that’s when I heard the big bang.”

Seeing billowing smoke, Heydinger located her girl and led her out of the confusion. Such instincts are personal — and professional. As executive director of the nonprofit Girls on the Run of Vermont program, she’s a guiding force for 3,000 third- through eighth-graders in 144 schools statewide.

Introducing youth raised on screens to fresh air, fitness and face time is a challenge specific to the 21st century.

“Girls on the Run is for girls of all abilities, sizes, shapes, different facets of life,” Heydinger says. “You don’t have to be athletic. It’s about coming together, making friends, having fun, learning about yourself, your goals and your values. It’s noncompetitive, so everyone feels accepted and bonds together.”

But the plight of girls who don’t see or seize their potential isn’t new. Heydinger rewinds back a half-century to the black-and-white era of “Mad Men,” when she was born into a household of three athletic boys.

“Then came me — my parents said, ‘She’s got to be a princess, so we’ll name her after Miss America.”

Nancy Anne Fleming won the title in 1960.

“The expectations for the first girl in our family were carved out: Just be pretty and polite.”

As early as elementary school, Heydinger shared another attribute with her namesake: She felt everyone was judging her.

“Little things can knock you down — it could be about your clothes or your hair, how tall or short you are, the money your family has or doesn’t have. The messages you get from peers, family, society and the media are always coming in. The way I felt about myself was shaped and solidified at an early age.”

Becoming shy and insecure, Heydinger discovered running and its ability to move her — personally as well as physically.

“It made me feel strong, competent and confident, and it gave me time to think.”

By her teenage years, she could jog as many as 10 miles.

“My mother followed me in the car to make sure I was OK, but that was a huge accomplishment for me.”

Heydinger earned a degree in education with hopes of working as a teacher. Then she took a temporary banking job that sprouted into a nearly two-decade financial career, all the while getting married and giving birth to two daughters.

Wanting her children to grow up with more self-confidence than she did, Heydinger learned of the national Girls on the Run program in 1999. Training with founder Molly Barker, she returned to her hometown of Vernon to start one of the first of the country’s 200 regional chapters.

Girls in two groups, grades 3-5 and 6-8, meet twice a week after school to explore three themes: Getting to know oneself (starting with physical and emotional health), teamwork (and related topics such as gossiping and bullying) and community (including public service and media literacy).

Participants then gather each spring for 5-kilometer runs (organized by her husband, Tom) in Brattleboro, Essex Junction and, new this year, Rutland.

“Getting them around this age is important,” Heydinger says. “Girls begin to confront important life and relationship issues and feel peer pressure but are still receptive to adult influence.”

Starting with her daughters and a dozen friends, Heydinger has expanded the program over the past 13 years to serve a total of 25,000 girls in every county except Essex in the Northeast Kingdom.

The only full-time employee, Heydinger has help from four part-time colleagues, 600 volunteer coaches (with more needed to accommodate a lengthening student waiting list) and supporters including Blue Cross and Blue Shield of Vermont, the state Agency of Education and Department of Health.

The program isn’t the only thing that keeps Heydinger running. Standing at the start of last month’s Boston Marathon, she decided the event would be her last. As such, she wondered: Should she stop and savor each moment or go all out? Donning headphones, she heard an inner voice: “Nancy, just see what you can do.”

Kicking off, Heydinger knew friends from Girls on the Run were set to cheer her on at mile 17, just before the hellish incline of Heartbreak Hill. But when she reached there, she didn’t see them. Perhaps they were at mile 18? Or 19? Or 20?

“I had three miles to go,” she recalls, “and my legs were screaming.”

Heydinger stopped at a medical tent to stretch. Glancing at her watch, she realized if she returned to the road, she could finish the marathon in less than four hours.

Grabbing her youngest daughter, Heydinger immediately drove back to Vermont. In the days and weeks since, she has fielded many questions from girls wondering about their own safety. She tells them the program always takes precautions. And she reminds them of its lessons on resilience.

“It’s never a race — we use physical activity to teach life skills. The real goal is to get girls to recognize and embrace who they are. It’s a wonderful place to come together to meet a challenge and celebrate. Running is such a gift. You can’t take away the joy.”

That’s why this year’s marathon may not be her last. Heydinger has completed 26-mile events with daughters Katy, a 24-year-old English teacher in China, and Caroline, a 22-year-old graduating senior at Georgetown University. But she has yet to try one with her son, Tommy, a 19-year-old freshman at Virginia Tech.

“I’m ready to be finished marathoning,” the mother says, “but if my kids want to run with me, how can I ever say no?”